Antennas may include transducers for electromagnetic waves and electric currents and the various shapes may have three complimentary forms: slot, panel and skeleton. For instance, the skeleton form of the circle antenna may include a circular wire loop, the complimentary panel structure may include a circular metal disc, and the slot structure may include a circular hole in a metal sheet. The various compliments are beneficial for different applications, such as realizing antennas of low wind resistance, antennas for an aluminum aircraft fuselage, or e.g. for metal stamping.
It is possible to have dual linear or dual circular polarization channel diversity. That is, a frequency may be reused if one channel is vertically polarized and the other horizontally polarized. Or, a frequency can also be reused if one channel uses right hand circular polarization (RHCP) and the other left hand circular polarization (LHCP). Polarization refers to the orientation of the E field in the radiated wave, and if the E field vector rotates in time, the wave is then said to be rotationally or circularly polarized.
Today, the antenna may be the only piece of associated equipment that remains to be miniaturized for use in various environments. Conformal antennas can be formed in situ from conductive surfaces, providing an antenna function without added size. For instance, a slot can be an antenna in the metallic structure of an aircraft without increasing the size of the aircraft or increasing drag. Although many slot antennas may be linear, e.g. a straight line in shape, the circular slot antenna may be advantaged: as the circle provides the greatest area for the smallest perimeter, it may provide the largest antenna aperture for the least circumference.
An electromagnetic wave (and radio wave, specifically) has an electric field that varies as a sine wave within a plane coincident with the line of propagation, and the same is true for the magnetic field. The electric and magnetic planes are perpendicular and their intersection is in the line of propagation of the wave. If the electric-field plane does not rotate (about the line of propagation) then the polarization is linear. If, as a function of time, the electric field plane (and therefore the magnetic field plane) rotates, then the polarization is rotational. Rotational polarization is in general elliptical, and if the rotation rate is constant at one complete cycle every wavelength, then the polarization is circular. The polarization of a transmitted radio wave is determined in general by the structure of the transmitting antenna, the orientation of the antenna, and the current distribution thereupon For example, the monopole antenna and the dipole antenna are two common examples of antennas with linear polarization. An axial mode helix antenna is a common example of an antenna with circular polarization, and another example is a crossed array of dipoles fed in quadrature. Linear polarization is usually further characterized as either vertical or horizontal. Circular Polarization is usually further classified as either Right Hand or Left Hand.
The dipole antenna has been perhaps the most widely used of all the antenna types. It is of course possible however to radiate from a conductor which is not constructed in a straight line. Preferred antenna shapes are often Euclidian, being simple geometric shapes known through the ages. In general, antennas may be classified as to divergence or curl of electric currents, corresponding to dipoles and loops, and line and circle structures.
Many structures are described as loop antennas, but standard accepted loop antennas are a circle. The resonant loop is a full wave circumference circular conductor, often called a “full wave loop”. The typical prior art full wave loop is linearly polarized, having a radiation pattern that is a two petal rose, with two opposed lobes normal to the loop plane, and a gain of about 3.6 dBi. Reflectors are often used with the full wave loop antenna to obtain a unidirectional pattern.
Dual linear polarization (simultaneous vertical and horizontal polarization from the same antenna) has commonly been obtained from crossed dipole antennas. For instance, U.S. Pat. No. 1,892,221, to Runge, proposes a crossed dipole system. A dual polarized loop antenna could be more desirable however, as loops provide greater gain in smaller area.
A slot form turnstile antenna is described in “A Shallow-Cavity UHF Crossed-Slot Antenna”, by C. A. Lindberg, Institute For Electrical and Electronics Engineers (IEEE) Transactions on Antennas and Propagation, Vol. AP-17, No. 5, September 1969. According to Lindberg, two dipoles are realized in sheet metal as crossed slots. The inside corners comprise 4 terminals that form 2 ports in a phase quadrature feed, e.g. 0, 90, 270, and 360 degrees at the terminals and 0, 90 degrees across the slots. Crossing dipoles and slot dipoles may be common for circular polarization, yet circular rather than X shapes may be advantaged for smaller size and greater directivity.
U.S. Pat. No. 5,977,921 to Niccolai, et al. and entitled “Circular-polarized Two-way Antenna” is directed to an antenna for transmitting and receiving circularly polarized electromagnetic radiation which is configurable to either right-hand or left-hand circular polarization. The antenna has a conductive ground plane and a circular closed conductive loop spaced from the plane, i.e., no discontinuities exist in the circular loop structure. A signal transmission line is electrically coupled to the loop at a first point and a probe is electrically coupled to the loop at a spaced-apart second point. This antenna requires a ground plane and includes a parallel feed structure, such that the RF potentials are applied between the loop and the ground plane. The “loop” and the ground plane are actually dipole half elements to each other.
U.S. Pat. No. 5,838,283 to Nakano and entitled “Loop Antenna for Radiating Circularly Polarized Waves” is directed to a loop antenna for a circularly polarized wave. Driving power fed may be conveyed to a feeding point via an internal coaxial line and a feeder conductor passes through an I-shaped conductor to a C-type loop element disposed in spaced facing relation to a ground plane. By the action of a cutoff part formed on the C-type loop element, the C-type loop element radiates a circularly polarized wave. Dual linear or dual circular polarization are not however provided.
U.S. Published Patent Application No. 2008 0136720 entitled “Multiple Polarization Loop Antenna And Associated Methods” to Parsche et al. includes methods for circular polarization in thin wire loop antennas. A full wave circumference loop is fed in phase quadrature (0°, 90°) using two driving points.
However, there is still a need for a relatively small planar and/or conformal slot antenna for operation with any polarization including linear, circular, dual linear and dual circular polarizations.